This invention relates to optical cavities. In particular, the present invention is a mirror mounting device and method for achieving optical alignment of mirrors assembled to form an optical cavity.
One embodiment of this invention is application to a ring laser gyroscope (RLG). A RLG is commonly used to measure the angular rotation of a vehicle, such as an aircraft. Such a gyroscope has two counter-rotating laser light beams which move within a closed loop path or "ring" with the aid of successive reflections from multiple mirrors. The closed path is defined by an optical cavity which is interior to a gyroscope frame or "block". In one type of RLG, the block includes planar top and bottom surfaces that are bordered by six planar sides that form a hexagon shaped perimeter. Three planar non-adjacent sides of the block form the mirror mounting surfaces for three mirrors at the corners of the optical path which is triangular in shape.
Operationally, upon rotation of the RLG about its input axis (which is perpendicular to and at the center of the planar top and bottom surfaces of the block), the effective path length of each counter-rotating laser light beam changes and a frequency differential is produced between the beams that is nominally proportional to angular rate. This differential is then measured by signal processing electronics to determine the angular rotation of the vehicle.
To produce accurate angular rate data (i.e., to minimize RLG bias errors), the counter-rotating laser light beams must be at substantially maximum intensity as they traverse the closed loop optical path defined by the optical cavity. To attain maximum light intensity, the counter-rotating laser light beams must be aligned within apertures of the optical cavity. However, a RLG block typically has unavoidable inaccuracies (otherwise known as "block geometry errors") that result from the block manufacturing process (i.e., the process of forming, machining and polishing the block). These geometry errors typically take the form of mirror mounting surface "tilt" with respect to the optical cavity apertures which define the desired closed loop optical path within the block. The tilted mirror mounting surfaces affect laser light beam alignment (or position) with respect to the apertures and thereby laser light intensity, causing inaccurate angular rate data. Therefore, a beam alignment process and/or apparatus is needed to properly position the counter-rotating laser light beams within the optical cavity.
Typically in a RLG block having a triangular shaped optical path incorporating three mirrors, one of the mirrors has a concave reflective surface while the other two mirrors have planar reflective surfaces. The curved mirror serves two main purposes. First, the curvature of the reflective surface controls the diameter and the primary mode of the counter-rotating laser light beams. Second, the curvature of the reflective surface is used to align the counter-rotating laser light beams within the optical cavity so that the light beams are at substantially maximum intensity to minimize RLG bias errors. In particular, this latter purpose is accomplished due to the inherent attributes of the concave reflective surface. By nature, the angle of the surface of a concave mirror varies in accordance with its curvature. Therefore, an incident laser light beam can be redirected or "steered" by translating (i.e., moving) the curved mirror within the plane of its respective block mounting surface.
In practice, with the two planar mirrors already mounted on the block, the concave mirror is translated to selectively steer the light beam within the optical cavity via a conventional mirror movement mechanism. During translation of the concave mirror, a detector, such as a photodiode, senses the intensity of the laser light output from the cavity through one of the planar mirrors that is partially transmissive. The photodiode generates an electrical signal representative of the intensity of the laser light output from the optical cavity. This signal is monitored by a voltmeter during such translations of the concave mirror until a mirror position is found exhibiting a maximum output on the voltmeter. This mirror position indicates that the counter-rotating laser light beams are at substantially maximum intensity and therefore are optimally aligned within the apertures of the optical cavity. The concave mirror is then secured to its mounting surface on the block at the optimum mirror position to complete the laser light beam alignment process.
Though the above described alignment mechanism and process adequately aligns the counter-rotating laser light beams within the optical cavity of the block so as to minimize RLG bias errors, there is at least one disadvantage. The mechanism and process described requires a great deal of handling of the concave mirror, particularly when translating the mirror about its mounting surface to identify the mirror's optimum mirror mounting position. The greater the extent of concave mirror manipulation, the better the chance of introducing contaminants (i.e., dirt) to or damaging the delicate reflective surface of the mirror. Any damage and/or contamination increases the likelihood of bias errors and degrades RLG performance. If the bias errors are too great and/or the RLG performance too corrupted, the RLG must be rebuilt or scraped. This increases the manufacturing cost of producing the RLG's.
There is a need for improved device and method for achieving optical alignment of an optical cavity such as the optical cavity of an RLG. In particular, there is a need for a mirror alignment device and method that reduces the amount of mirror handling needed align the light beams within the optical cavity. In addition, the device and method should reduce the likelihood of mirror reflective surface damage and/or contamination during alignment, to reduce the number of RLG's needing to be rebuilt or scraped. Lastly, the mirror alignment device and method should be relatively easy and inexpensive to practice and should greatly facilitate automation of assembly.